To limit pollutant emissions, modern motor vehicles powered by internal combustion engines are equipped with fuel vaporization restraint systems, commonly referred to as tank venting devices. The purpose of such devices is to accommodate and temporarily store fuel vapor that forms in a fuel tank as a result of vaporization, so that the fuel vapor cannot escape into the environment. As storage for the fuel vapor, a fuel vapor retention filter is provided in the fuel vaporization restraint system; this uses, for example, activated carbon as a storage medium. The fuel vapor retention filter has only a limited storage capacity for fuel vapor. In order to be able to use the fuel vapor retention filter over a long period of time, it must be regenerated. For this purpose, a controllable tank venting valve is arranged in a line between the fuel vapor retention filter and an intake manifold pipe of the internal combustion engine, which valve is opened for executing the regeneration, so that on the one hand the fuel vapors adsorbed in the fuel vapor retention filter escape into the intake manifold pipe as a result of the reduced pressure in the latter, and so are fed into the intake air of the internal combustion engine, and thus to the combustion process, and on the other hand, the adsorption capacity of the fuel vapor retention filter for fuel vapor is restored.
A regeneration process of the fuel vapor retention filter is therefore only possible if a reduced pressure prevails in the intake manifold pipe relative to the tank venting device. New vehicle concepts with hybrid drive and start/stop functionality are a means of complying with legislated emission values and reducing fuel consumption. At the same time, however, these lead to a significant reduction in the scavenge rates for the regeneration of the fuel vapor retention filter, since the effective time in which the internal combustion engine can be scavenged is reduced by the temporary shutdown of the internal combustion engine. Furthermore, the dethrottling of the internal combustion engines as a result of the elimination of the throttle valve, and the control of the incoming airflow using the intake valves (VVT, variable valve train) and/or exhaust gas turbocharging leads to the fact that the reduced pressure in the intake manifold pipe required for the scavenging of the fuel vapor retention filter is no longer sufficient.
In DE 10 2010 054 668 A1 an internal combustion engine is described with a fuel tank, a fuel vapor store for storing fuel vapors that escape from the fuel tank, a connecting line between the fuel vapor store, and an air intake manifold of the internal combustion engine to lead fuel vapors from the fuel vapor store into the air intake manifold during a regeneration phase, a valve arranged in the connecting line, a venting line for the fuel vapor store and a valve unit arranged in the venting line for controlling the venting of the fuel vapor store. A scavenge air pump is arranged in the venting line for the fuel vapor store; this is integrated into the valve unit for controlling the venting of the fuel vapor store. In this way, a particularly effective scavenging and regeneration of the fuel vapor store is achieved, even if no or a slightly reduced pressure, is provided by the air intake manifold.
During the tank venting process, an additional fuel fraction enters the combustion chamber of the internal combustion engine from the fuel vapor retention filter when the gas inlet valve is open. In order to ensure correct operation of the internal combustion engine and compliance with exhaust gas limits, this fuel fraction must be taken into account in the total amount of fuel to be supplied as calculated by the engine control unit for the instantaneous operating point of the internal combustion engine. For the control of the scavenge flow and the injection correction, knowledge that is as accurate as possible of the vaporous fuel fraction (HC/air mixture from the fuel vapor retention filter), that is to say, the degree of loading of the fuel vapor retention filter, is thus necessary.
The determination of the degree of loading is carried out in conventional systems by evaluating the signal deviation of a lambda probe arranged in the exhaust gas manifold upstream of an exhaust gas catalytic converter as the tank venting valve is slowly opened. Since deviations of the lambda probe signal can also be attributed to other causes, for example, as a result of a load change, determination of the degree of loading based on this signal deviation may lead to erroneous results. The consequence of this is an erroneous calculation of the quantity injected, which can lead to increased exhaust emissions, increased fuel consumption and poorer drivability. In addition, only very little HC gas can be regenerated during this relatively long learning phase.
A method and a device for the operation of a tank venting system for an internal combustion engine are described in EP 2 627 889 B1. The tank venting system has an adsorption tank, a regeneration passage, and an electrically driven pump. The adsorption tank is used for the collection and intermediate storage of fuel vapors emerging from a fuel tank, wherein a scavenge airflow can flow through the adsorption tank. The regeneration passage connects the adsorption tank with an intake passage. A pump is arranged in the regeneration passage, which is designed to suck the scavenge air out of the adsorption tank and to add it to an intake air in the intake passage. A density of the scavenge air flowing in the regeneration passage is determined. Furthermore, a scavenge air mass flow rate flowing in the regeneration passage is determined, which is dependent on the density of the scavenge air and a predetermined pump characteristic of the pump.